Laminated ceramic circuit substrates having internal layer circuits, onto which semiconductor elements such as ICs, etc. and various electronic parts are mounted, have conventionally been known. Such laminated circuit substrates have conventionally been made of alumina having excellent heat dissipation, electric properties, mechanical strength, etc. as a whole. Because alumina substrates have as high sintering temperatures as 1300-1600° C., high-melting-point metals such as W, Mo, etc. are used for electrodes. However, these electrode materials have high electric resistivity, suffering from large signal transmission loss in high-frequency circuits of more than several hundreds of MHz in cell phones, etc.
In a mobile communications field of cell phones, etc., a small signal transmission loss is particularly required. Accordingly, electrode materials having small electric resistivity such as silver, gold, copper, etc. have come to be used in place of high-melting-point metals such as W, Mo, etc., and glass ceramics and low-temperature-sintered ceramic compositions simultaneously sintered with the electrode materials have come to be widely used for laminated ceramic circuit substrates (U.S. Pat. No. 6,121,174).
However, the laminated circuit substrates are extremely poorer in mechanical strength than the alumina substrate. For instance, the alumina substrate has a bending strength of about 400 MPa, while the laminated circuit substrates have a bending strength of about 150 MPa. The bending strength of 150 MPa or more is sufficient for practical use in the laminated circuit substrates in conventional cell phones, etc. However, as electronic parts used in cell phones, etc. become thinner, laminated circuit substrates used therein also become as thin as 1 mm or less, so that conventional laminated ceramic circuit substrates fail to provide sufficient mechanical strength.
Thus, higher strength is required for laminated circuit substrates used in cell phones, etc., such that cracking and breakage are not generated, for instance, by the deformation such as twisting, bending, etc. of mounting substrates or by impact when dropped.